The present invention relates to a method and apparatus for forming an excellently crystalline dielectric film with a high dielectric constant, like a CeO2 film, or ferroelectric film out of metal Ce and oxygen on an Si substrate.
In recent years, the number of C-MOS devices that can be integrated together on a single Si substrate has increased significantly because those devices have been tremendously downsized. To catch up with this trend, reduction in thickness of a gate insulating film, which is part of a MOSFET, is also in high demand. A thinner gate insulating film is needed because of the following reasons.
First of all, although the operating voltage has been reduced day after day to conserve power as much as possible, the quantity of charge required for the operation of a device remains almost the same and has not been reduced so much. Since the relationship of Q=CV (where Q is the quantity of charge, C is static electricity and V is voltage) should be met, the static electricity C that can be retained in a gate insulating film must be increased to reduce the voltage V with the quantity of charge Q kept substantially constant. The static electricity C is given by C=(∈rxc2x7S)/d, where ∈r is relative dielectric constant, S is the area of a capacitor and d is a space between electrodes. Accordingly, the static electricity C can be increased if the thickness d of a gate insulating film, which is currently made of SiO2 in many cases, is reduced. For that purpose, the gate insulating film has been thinned to a thickness between 10 and 15 nm or less than 10 nm.
However, if a gate insulating film is thinned that much, then various inconveniences might be concerned about lately; the breakdown strength of the gate insulating film might decrease or the leakage current might increase.
In view of these potential disadvantages, alternative gate insulating film materials, which have a relative dielectric constant ∈ r higher than that of SiO2 and yet exhibit pretty good electrical properties comparable to those of SiO2, have been searched for. That is to say, if the relative dielectric constant ∈ r is higher, then the static electricity C can be kept high even when the thickness d is increased to a certain degree. Accordingly, the required charge quantity Q is attainable even with a reduced operating voltage. Taking these points into account, methods for forming, on an Si substrate, an insulating film made of a novel insulating material with high dielectric constant and breakdown strength and low interface level and leakage current have been researched and developed to attain characteristics comparable to those of the SiO2 gate insulating film currently used.
Efforts have also been made to form an insulating film of non-SiO2 insulator material on an Si substrate by a different type of demand. For example, an example disclosed in Japan Journal of Applied Physics 35, 4987, (1996) (which will be herein called a xe2x80x9cfirst documentxe2x80x9d for convenience sake) report-research for implementing a transistor with memory function by providing a thin film with ferroelectric for the gate of field effect transistor. According to the technique disclosed in this document, a thin film of PbZr1-xTixO3 (PZT) with ferroelectric, i.e., a PZT film, is formed as an exemplary thin film of that type. However, since it is difficult to form the PZT film directly on an Si substrate, an insulating film of CeO2 for example, is interposed as a buffer layer between the PZT film and the Si substrate.
Methods for forming a novel insulator film on an Si substrate to attain those characteristics, including high dielectric constant and breakdown strength and low interface level and leakage current, as in the gate insulating film mentioned above, have also been researched such that a ferroelectric or other dielectric film (e.g., superconductor film) can be formed on the Si substrate.
According to any of these suggested techniques, a CeO2 film is one of very attractive insulator materials for a buffer layer. This is because the lattice constant of CeO2 is closer to that of Si than any other known material and a lattice mismatch between CeO2 and Si is only xe2x88x920.37% (i.e., aceO2=5.411 xc3x85 and asi=5.431 xc3x85). In addition, since the crystal structure of CeO2 is like that of fluorite, CeO2 can form a continuous crystal lattice with the Si substrate having a diamond structure. The coordination number for all the atoms is four in Si, whereas the coordination number for oxygen atoms is four and that for Ce atoms is eight in CeO2. However, since both Si and CeO2 crystals belong to a cubic system, which is represented as a face-centered cubic lattice as a matter of principle, Si and CeO2 crystals can be stacked one upon the other by epitaxial growth (because mole ratio of oxygen to Ce is 2:1). Thus, it is possible to form a thin film with excellent crystallinity on the Si substrate, and it is easier to stack a ferroelectric or superconductor film with high crystallinity thereon. Furthermore, since the relative dielectric constant of CeO2 is as high as around 26, it is very likely that CeO2 will be used as a novel gate insulating film material in place of SiO2.
Various techniques of forming CeO2 on an Si substrate have been proposed in numerous other documents as well as the first document. Following is typical examples of them.
According to an example disclosed in Japan Journal of Applied Physics, 1765, (1993) (which will be herein called a xe2x80x9csecond documentxe2x80x9d for convenience sake), CeO2 is evaporated from a pellet-like CeO2 sintered compact by irradiating the compact with an electron beam (EB) in a molecular beam epitaxy (MBE) system including an EB evaporation unit, thereby forming an excellently crystalline CeO2 thin film on an Si substrate. In this case, decline in crystallinity of the CeO2 thin film due to the oxygen deficiency is prevented by supplying oxygen gas while CeO2 is being evaporated. In the first document, the CeO2 film is also formed by the same method.
An example disclosed in Japan Journal of Applied Physics 270, 1994 (which will be herein called a xe2x80x9cthird documentxe2x80x9d for convenience sake) uses a thin film forming technique different from that of the first and second documents. In the third document, a reactive sputterer including a target of metal Ce is used and Ce atoms are sputtered out of the target with oxygen gas supplied thereto and reacted with oxygen on the Si substrate, thereby forming an excellently crystalline CeO2 thin film on the Si substrate.
An example disclosed in Applied Physics Letters 2027, (1991) (which will be herein called a xe2x80x9cfourth documentxe2x80x9d for convenience sake) forms a CeO2 film by a different technique from any of the techniques mentioned above. In the fourth document, an MBE system, into which ArF excimer laser radiation can be introduced externally, is used, a pellet-like CeO2 sintered compact placed inside is irradiated with the laser radiation to evaporate CeO2 therefrom and oxygen gas is introduced at the same time. In this manner, an excellently crystalline CeO2 thin film is formed on the Si substrate.
These methods of forming a crystalline CeO2 thin film as disclosed in the documents cited above, however, have the following shortcomings.
It should be noted that a family of crystallographic planes including (100), (010), (001) and so forth will be collectively referred to as a (001) plane in the following description, although such a family of planes should be labeled {001}. The same statement will be applicable to a (011) or (111) plane. Similarly, (001), (011) or (111) substrate or film will mean a substrate or film with a (001), (011) or (111) plane as its principal surface.
First, in accordance with the example disclosed in the first and second documents, oxygen and Ce are supplied at the same time by evaporating CeO2 from a pellet-like CeO2 sintered compact being heated. That is to say, since Ce and oxygen reach the surface of the Si substrate at a time, SiO2, as well as CeO2, is formed thereon. Should SiO2 be formed, the sharpness of crystallinity decreases at the interfacial structure and the planarity of the surface also deteriorates since SiO2 generally has an amorphous structure. Also, if the structure with the SiO2 film is operated as a device, a voltage applied will be concentrated on the SiO2 film with the lower dielectric constant in spite of the presence of the CeO2 film with the higher dielectric constant. As a result, it is difficult to store charge to a quantity large enough to ensure the intended function of a gate insulating film. In addition, even when such a CeO2 film mingled with SiO2 is used as a buffer layer for a ferroelectric or superconductor film, it is also difficult to apply a required voltage to the ferroelectric or superconductor layer.
Following the example disclosed in the second document, a CeO2(111) film can be formed on an Si(111) substrate. But only a CeO2(011) film, not a CeO2(001) film, can be formed on an Si(001) substrate. That is to say, even though the lattice constants of CeO2 and Si are close to each other, the plane orientations thereof are different from each other. Thus, the effects of suppressing lattice strain and preventing the generation of defects cannot be expected at all. Furthermore, although a CeO2(011) film is formed, the film actually has a polycrystalline structure, in which two types of crystals coexist on the principal surface of the Si substrate so as to be symmetrical around an axis and form an angle of 90 degrees between them. Accordingly, it is difficult to obtain a smooth and uniform single crystal thin film.
Japan Journal of Applied Physics 31, L1736, (1992) (which will be herein called a xe2x80x9cfifth documentxe2x80x9d for convenience sake) explains the reason of this phenomenon. Specifically, it is believed that higher stability is attained where the (001) plane of Si crystals is continuous with the (011) plane of CeO2 crystals rather than where the (001) planes of these two types of crystals are continuous with each other. This is probably because dangling bonds appearing on the 2xc3x971 streaks on the (001) plane of Si crystals that are formed in high vacuum are located close to oxygen atoms within the (011) plane of CeO2 crystals.
The fourth document shows that a CeO2(111) film with excellent crystallinity can be formed on an Si(111) substrate. In accordance with this example, an oscillation of diffraction pattern intensity (RHEED oscillation) is observed by a reflection high-energy electron diffraction (RHEED) analysis during the crystal growth. The generation of the RHEED oscillation indicates that the crystals are growing two-dimensionally, or layer by layer, while keeping high smoothness at the surface. Even when the cross section thereof is observed by TEM, the existence of large defects is hardly observable. The formation of SiO2 is not found in the interface between Si and CeO2, either. However, this document does not report on successfully forming a (001) plane of CeO2 crystals on a (001) plane of Si crystals, either.
Japan Journal of Applied Physics 29, L1199, (1990) (which will be herein referred to as a xe2x80x9csixth documentxe2x80x9d for convenience sake) makes a disclosure about this. Specifically, since Ce and oxygen are also supplied at a time even by using such a system, a CeO2(011) film is formed unintentionally on an Si(001) substrate.
Even in the example disclosed in the third document, a (111) plane of CeO2 with excellent crystallinity is formed on a (111) plane of an Si substrate as in the second and fourth documents. In accordance with the method disclosed in the third document, metal Ce is used as a source material. Thus, the method succeeds in supplying only Ce onto the interface of the Si substrate and suppressing the formation of SiO2. However, a layer consisting, of metal Ce alone, which is needed in obtaining the excellently crystalline CeO2 film, is as thick as 5 nm. Accordingly, if the film is used as a gate insulating film for a transistor, then the operation of the transistor device is seriously affected by the existence of the thick metal layer. In addition, the third document does not report on successfully forming a CeO2(001) film on an Si(001) substrate, either. The reason thereof cannot be found in the document. But we think it would be difficult to form CeO2 crystals with the same plane orientation by relaying information about the crystal structure of the Si substrate if the metal Ce layer with the thickness of about 5 nm exists between them.
An object of the invention to work out a technique of forming an excellently crystalline metal oxide layer, such as a CeO2 film, on an Si substrate, which has been impossible according to any of the documents cited, and thereby develop a technique of forming a ferroelectric film with good orientations on the crystalline metal oxide layer.
First, it will be described what analyses we carried out to arrive at the inventive idea on the novel method for forming a metal oxide layer like a CeO2 film.
The fifth document discloses the reasons why only a CeO2(011) film, not a CeO2(001) film, can be formed on an Si(001) substrate. Those reasons will be described in detail below.
FIG. 19 illustrates how CeO2 crystals grow epitaxially on a (001) plane of an Si substrate and corresponds to FIG. 2 of the fifth document. In FIG. 19, the larger open circles represent Ce atoms 1, the medium dotted circles represent Si atoms 2 and the smaller open circles represent oxygen atoms 3. On the surface of the Si substrate, a (001) plane of Si crystals with a unit cell 4 has appeared. One side of the unit cell 4 is parallel to the [100] direction, while another side of the unit cell 4 is parallel to the [010] direction. In other words, supposing the paper of FIG. 19 is a plane parallel to the (001) crystallographic plane of Si crystals, the x- and y-axes of Si crystals exist within the paper and the z-axis of Si crystals is the direction coming out of the paper. On the other hand, two types of CeO2 crystals, which are represented by two unit cells 5 and 6 shown in FIG. 19, are created at an equal percentage to match the (001) plane of Si crystals. One side of each of these unit cells 5 and 6 is parallel to the [100] direction (i.e., the x-axis direction), while another side thereof is parallel to the [011] direction (i.e., the direction tilted from the y-axis by 45 degrees). The x-axes of the unit cells 5 and 6 cross each other at right angles, and the [011] direction of the unit cell 5 and the [011] direction of the unit cell 6 also cross each other at right angles. In other words, the unit cell 6 is obtained by rotating the unit cell 5 by 90 degrees around an axis in the direction coming out of the paper of FIG. 19. The y- and z-axes of each of the unit cells 5 and 6 are tilted from the paper of FIG. 19 by 45 degrees.
As shown in FIG. 19, the O atoms 3 are located over and between columns of Si atoms 2 in the Si crystals in the (011) plane of CeO2 crystals. When viewed one-dimensionally along the [100] direction of the lattice structure of Si crystals, i.e., when viewed in the direction coming out of the paper, these O atoms 3 are located very close to the dangling bonds appearing on the 2xc3x971 restructured structure on the uppermost surface of the Si substrate. Accordingly, Ce and O (oxygen), which are supplied at a time onto the Si substrate, are likely to form the (011) plane of CeO2 crystals rather than a (001) plane of the CeO2 crystals. And when the (011) plane of CeO2 crystals is formed on the Si substrate, two types of crystal structures, i.e., the unit cells 5 and 6, appear at an equal percentage to be symmetrical to each other around an axis as shown in FIG. 19. Thus, when CeO2 crystals are grown epitaxially on the Si substrate, two types of crystals with two different orientations coexist and form respective domains. As a result, a CeO2 film, which is regarded as polycrystalline as a whole, is formed.
FIG. 20 is a microgram obtained by observing the coexistence of two domains in a CeO2 film by high-resolution transmission electron microscopy (HRTM) as disclosed in the fifth document.. As shown in FIG. 20, a domain CrA with an x-axis in the [100] direction parallel to the horizontal direction of FIG. 20 and a domain CrB with an x-axis in the [100] direction parallel to the vertical direction of FIG. 20 coexist. The sizes of these domains CrA and CrB are in the range from 10 to 50 nm.
FIG. 18 is a schematic cross-sectional view illustrating a state where two types of CeO2 crystal domains exist in a CeO2(011) film 9 formed on an Si(100) substrate 8.
Next, a process through which CeO2(111) crystals are formed on an Si(111) substrate will be considered. The structures of Si and CeO2 crystals in such a case are disclosed in the sixth document. FIGS. 21(a) through 21(c) correspond to FIG. 4 of the sixth document and illustrate orientations of CeO2 crystals, which grow epitaxially on (001), (111) and (110) planes of an Si substrate. FIG. 21(a) illustrates that two types of CeO2 crystal domains, whose film plane is a (011) plane, are formed on the (001) plane of the Si substrate as in the fifth document. On the other hand, FIG. 21(b) illustrates that CeO2(011) and (111) films can be formed on an Si(011) substrate.
As can be seen from FIG. 21(b), the CeO2(111) film is likely to grow on the Si(111) substrate and there is little lattice mismatch therebetween. In this case, the CeO2 crystals have such a structure as including a layer consisting of Ce alone and a layer consisting of oxygen alone that are alternately stacked one upon the other vertically to the surface of the substrate, strictly speaking. However, since these two layers are located very close to each other, two types of atoms can be regarded as substantially existing in a common plane. Thus, the energy required for eliminating one of these two types of atoms, i.e., Ce and O atoms, and thereby forming a layer consisting of only one remaining type of atoms is not high for any layer of either type of atoms. That is to say, even if Ce and O atoms are supplied at a time onto the Si substrate, the CeO2(111) film can still be formed. And we can say that no other CeO2 film with a different plane orientation will be formed.
However, since the (111) plane is the densest plane of crystals making up a diamond structure, the greatest number of Si dangling bonds exist on the (111) plane. Accordingly, if O and Ce atoms are supplied at a time onto that surface of the Si substrate, then not only CeO2 crystals but also an SiO2 layer will be formed on the surface of the Si substrate. That is to say, crystallinity might decline and dielectric constant might decrease.
In the first, third, fourth and sixth documents, the crystallinity of a CeO2 thin film obtained is analyzed with X-rays. Among these XRD analysis curves, that of the fourth document represents a diffraction peak with the smallest full width of half maximum (FWHM), which is still as large as 3500 arc sec., though. The FWHMs shown in the other documents are much greater than that. This is a very bad value considering that the lattice mismatch ratio between Si and CeO2 crystals is only xe2x88x920.37%. For example, the FWHM of ZnSe, which has a lattice mismatch ratio of 0.26% with respect to GaAs (where aGaAs=5.6533 and aZnSe=5.668), is 300 arc sec. or less (as for a 2xcex8-axis-fixed, xcfx89-axis-scanning rocking curve). The FWHM values shown in the respective documents are obtained by performing scanning on both axes of xcfx89xe2x88x922xcex8 (xcex8xe2x88x922xcex8). Thus, supposing half of each of these values is equivalent to the value obtained by xcfx89-axis scanning, the largest FWHM is still about 6 times as large as the FWHM of ZnSe. That is to say, the CeO2 film according to these documents seems to be smooth and less defective at a local level observable by TEM or the like. In the CeO2 film, however, significant lattice disorder and irregularity like defects are found within a range corresponding to a spot diameter of a beam of X-rays. Accordingly, compared to crystals used for a compound semiconductor, the crystallinity thereof seems to be much inferior. As can be seen, such poor crystallinity of the CeO2 film seems to be one of the reasons why the SiO2 layer is formed unintentionally between the Si substrate and the CeO2 film. Specifically, if the lattice is out of order in the CeO2 layer already formed, then oxygen (O) atoms are easily transmitted through those disordered parts. As a result, a greater number of oxygen atoms are supplied onto the surface of the Si substrate. In addition, a great number of dangling bonds also exist on parts of the surface of the Si substrate just under the disordered parts of the CeO2 crystal lattice, and therefore oxygen is easily bonded to those parts, thus promoting the formation of the SiO2 layer.
In view of these findings, the present inventors arrived at the idea that if monatomic Ce and O layers are stacked one upon the other on an Si(001) substrate with Ce and oxygen alternately supplied onto the Si(001) substrate and with Ce, La and oxygen supplied in controlled amounts, then a CeO2(011) film with double domains will not be formed but a single crystal CeO2(001) film can be formed. That is to say, we paid special attention to the fact that when crystals of a CeO2(001) film with a fluorite-type crystal structure grow, a layer where only Ce atoms exist and a layer where only oxygen atoms exist will appear alternately and repeatedly at regular intervals in the crystal lattice.
Taking these results into consideration, we also acquired the idea that if Ce and oxygen are alternately supplied after a thin film having a structure continuous with the diamond cubic structure of Si (e.g., an oxide with a fluorite-type crystal structure) has been formed on the Si(001) substrate, then a CeO2(011) film with double domains will not be formed but a single crystal CeO2(001) film can be formed.
Furthermore, we got the idea that even a metal oxide layer containing a metal other than Ce can be formed to show excellent crystallinity by taking measures for suppressing reaction between Si and O and that a ferroelectric layer with good orientations should be formed on the metal oxide layer.
Hereinafter, the present invention, which is a natural result of these findings, will be described.
A first inventive dielectric film forming method includes the steps of: (a) preparing a substrate including a crystalline semiconductor layer; (b) forming an underlying layer, which consists essentially of a metal material, on the crystalline semiconductor layer; (c) forming a metal oxide layer by oxidizing at least part of the underlying layer with oxygen supplied thereto from above the underlying layer; and (d) forming a ferroelectric layer on the metal oxide layer.
According to this method, an excellently crystalline underlying layer is formed while preventing an oxide film from being formed due to oxidation of a semiconductor material. Thus, a ferroelectric layer with good orientations is formed on the metal oxide layer, which part or all of the underlying layer. Consequently, a dielectric film qualified for a ferroelectric device can be obtained.
The step (c) may be performed in parallel with the step (d) by using oxygen supplied in the step (d) to form the ferroelectric layer.
The underlying layer formed in the step (b) preferably consists essentially of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi.
The method may further include the step of forming a thermal oxide film by thermally oxidizing a surface of the crystalline semiconductor layer after the step (a) and before the step (b). In the step (b), the underlying layer consisting essentially of the metal material may be formed on the thermal oxide film. In this manner, a thermal oxide film showing high affinity for the semiconductor layer can be used and yet the thermal oxide film, underlying layer and ferroelectric layer can be formed with almost no interdiffusion caused among them. Consequently, a dielectric film qualified for a ferroelectric memory device can be formed easily.
A second inventive dielectric film forming method includes the steps of: (a) preparing a substrate including a crystalline semiconductor layer; (b) forming an underlying layer, which consists essentially of a metal material, on the crystalline semiconductor layer, where affinity of the metal material for oxygen is higher than affinity of a semiconductor material of the semiconductor layer for oxygen; (c) forming a Ce layer on the underlying layer; and (d) forming at least a CeO2 layer by supplying oxygen from above the Ce layer.
According to this method, an excellently crystalline CeO2 layer can be formed while preventing an oxide film from being formed out of the semiconductor material. Thus, even if the dielectric film is relatively thick, the dielectric film has large capacitance per unit area, small leakage current and high breakdown strength.
In the step (d), at least part of the underlying layer is preferably oxidized.
The steps (b) through (d) are preferably performed continuously within an ultrahigh vacuum epitaxial growth system.
The steps (b) and (c) are preferably performed by an MBE process using an EB evaporation system.
The underlying layer formed in the step (b) preferably consists essentially of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi.
A third inventive dielectric film forming method includes the steps of: (a) preparing an Si substrate; and (b) forming a CeO2 film on the Si substrate by an epitaxy process using metal Ce as a source material.
According to this method, Ce can be supplied independently onto the surface of an Si substrate before oxygen is supplied thereto. Thus, it is possible to suppress the formation of an SiO2 layer on the surface of the Si substrate. For example, by evaporating and supplying metal Ce using a K-cell or a valved cracking cell, a Ce layer can be deposited at a deposition rate of 5 nm/min. or less. As a result, its thickness is controllable with a precision corresponding to the thickness of several atomic layers (5 xc3x85) or less, and therefore, a CeO2 film with a desired plane orientation can be formed.
If the principal surface of the Si substrate is a (001) plane, then a surface of the CeO2 film can also be a (001) plane. Thus, it is possible to prevent the generation of double domains, which are observed when a CeO2 film with a (011) film surface is formed. As a result, an excellently crystalline CeO2 film can be formed.
In the step (b), at least one atomic layer of Ce may be formed on the Si substrate. Then, it is possible to prevent an SiO2 layer from being formed on the Si substrate with more certainty.
In the step (b), Ce alone and oxygen alone may be alternately and repeatedly supplied such that monatomic O layers and monatomic Ce layers are formed alternately on the at least one Ce atomic layer located on the Si substrate. In this manner, it is possible to prevent a CeO2 film with a (011) surface from being formed.
A first inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a metal material, the affinity of the metal material for oxygen being higher than affinity of a semiconductor material of the semiconductor layer for oxygen; and a crystalline CeO2 film formed on the underlying layer.
In this structure, an oxide film of a material for a semiconductor layer, e.g., a silicon dioxide film, is less likely to be formed on the semiconductor layer. Thus, a dielectric film with a large capacitance per unit area can be formed to include an underlying layer with a high dielectric constant and a crystalline CeO2 layer. Consequently, the leakage current can be reduced and the breakdown strength can be increased by thickening the dielectric film sufficiently.
In the first dielectric film, at least part of the underlying layer has preferably been oxidized.
A second inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a compound oxide containing a metal element and a semiconductor material for the semiconductor layer; and a crystalline CeO2 layer formed on the underlying layer.
In this manner, a dielectric film, which includes: an underlying layer showing high affinity for the semiconductor layer, low distortion and low interface levels; and a CeO2 layer with a high dielectric constant, can be obtained.
A third inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a crystalline metal oxide that substantially lattice-matches crystals of the semiconductor on the principal surface of the semiconductor layer; and a crystalline CeO2 layer formed on the underlying layer.
In this manner, a dielectric film including an excellently crystalline CeO2 layer, which has reflected information about the crystal structure of a semiconductor layer by way of an underlying layer, can be obtained.
A fourth inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi; and a ferroelectric layer formed on the underlying layer.
In this manner, an underlying layer including almost no oxide film of the semiconductor material, e.g., silicon dioxide film, and a ferroelectric layer with good orientations can be obtained. Thus, it is possible to effectively impart a voltage applied to the entire dielectric film to the ferroelectric layer. Consequently, the ferroelectric layer attains sufficiently large residual polarization.
In the fourth dielectric film, at least part of the underlying layer has preferably been oxidized.
A fifth inventive dielectric film includes: an oxide layer formed on a crystalline semiconductor layer and made of an oxide of a semiconductor material for the semiconductor layer; an underlying layer formed on the oxide layer and made of an oxide of a metal material; and a ferroelectric layer formed on the underlying layer.
A layer made of an oxide of a semiconductor material for a semiconductor layer shows low interface levels and very high affinity for the semiconductor layer. By taking advantage of these properties, a very reliable dielectric film including a ferroelectric layer with good orientations can be obtained.